Many applications exist in which the accurate measurement of one or more dynamic volumes is desirable. For example, in the field of physiology, medical doctors, researchers and others use volumetric measurements to monitor and characterize physiological functions within a subject (e.g., a human patient), and diagnose ailments thereof.
One specific example includes the measuring of breathing volumes in patients. Respiratory disease is a common and significant problem in both the United States and throughout the world. Obstructions can stem from the constriction of the airways caused by inflammation and edema of the walls of the terminal bronchi, or narrowing of the trachea or the throat. Obstructions in the distal airways generally make expiration slower and/or more difficult as more air is exhaled from the lungs or during the latter part of expiration, while upper airway obstruction might provide a more constant resistance to air flow. Types of respiratory disease include diseases of the lung, bronchial tubes, pleural cavity, upper respiratory tract, trachea, and of the nerves and muscles of breathing. An important step in monitoring for and managing such diseases, as well as less severe respiratory conditions, involves measuring air flow volumes moving into and out of the patient's lungs.
Recording breathing volumes is commonly performed through the use of a volume flow-sensing device connected to a subject's airway (e.g., a spirometer or tachymeter), although such devices can be overly intrusive. Another method for measuring breathing volumes includes measuring the movements of the subject's chest and abdominal walls. These techniques are often strain gauge based (i.e., determining changes in body circumference) or based on elastic inductive electrical conductor loops arranged around the chest and abdomen of the subject. Changes in body circumference, or recordings of the inductance of the loops, can then be used to estimate the magnitude of cross sectional area and volume variations of the chest and abdominal compartments.
The strain gauge or circumferential distance methods have no simple or reproducible relation between the measured variations and the volumes that are measured. This relation depends on assumptions about the relation between the area enclosed by the loop and the length of the loop that are valid only for a fixed geometry. Although some of the methods based on inductance may claim that area is measured (i.e., it is assumed to be proportional to loop inductance), the assumption is only valid as long as the relative shape of the loop is conserved. Unfortunately, this is not the case for the cross-sectional area variations of, e.g., the human chest or abdomen that occur during respiration.
In response to these and other drawbacks of past methods of measuring volumes, VoluSense, the assignee of the present invention, has developed new systems and methods for measuring volumes, including the use of a volume-sensing element and associated electromagnetic induction techniques. Some of these systems and methods are disclosed in VoluSense's U.S. Pat. Nos. 6,374,667; 6,945,941; and 7,390,307, the contents of which are hereby incorporated by reference herein in their entirety.
The electromagnetic induction techniques described in these patents provide a measurable advance over past methods of measuring volumes. However, VoluSense continuously looks for ways to optimize equipment, methods, and other aspects of its existing systems, as will become apparent throughout the remainder of this disclosure.